Feeling suffocated doesn't usually happen overnight. It usually builds up. When I'm in a relationship that starts to feel that way, I will say something. Unfortunately, the other person usually either doesn't get it or can't help it. At that point, I just need it to STOP which means cutting off all contact. Being around someone who makes you feel smothered can actually be really frightening-- it makes me feel like I cant breathe and I can't escape and I just want to lock myself in a room in order to feel safe. When someone is making you feel that way, you don't just want a little bit less of them. You want to completely cut ties. Whatever you initially liked about that person goes away and the thought of that person just makes you want to hide. People need space and room to desire someone-- if you aren't giving that room, the desire will die.